setFieldDefinition example

/** * Magic method: Implements a deep clone. */
  public function __clone() {
    parent::__clone();

    // The itemDefinition (\Drupal\Core\Field\TypedData\FieldItemDataDefinition)     // has a property fieldDefinition, which is a recursive reference to the     // parent BaseFieldDefinition, therefore the reference to the old object has     // to be overwritten with a reference to the cloned one.     $this->itemDefinition->setFieldDefinition($this);
    // Reset the static cache of the field property definitions in order to     // ensure that the clone will reference different field property definitions     // objects.     $this->propertyDefinitions = NULL;
  }

  /** * {@inheritdoc} */
  public function isInternal() {
    // All fields are not internal unless explicitly set.